Why does my Nest say it is heating but the radiators are cold?

The thermostat is calling for heat but the signal is not reaching the boiler. This is usually a faulty receiver relay, a wiring issue, or a stuck motorised valve.

How do I know if the receiver is working?

When the thermostat calls for heat, you should hear a click from the receiver and see an indicator light. If there is no click or light, the receiver is not responding.

Could the wiring be wrong?

If the system never worked correctly since installation, a wiring error is likely. Even a single wire in the wrong terminal can prevent the system from controlling the heating.

Should I replace the thermostat or the receiver?

An electrician can quickly test both components. The receiver (which contains the relay) is more commonly the point of failure than the thermostat itself.

Low risk

Updated 30 August 2026

Smart Thermostat Not Controlling Heating

Your smart thermostat appears to work but the heating does not respond to it.

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What's happening?

When your smart thermostat shows it is calling for heat but the heating does not come on (or vice versa), the thermostat is not properly controlling the heating system. The thermostat display may look normal, the app may work, but the actual heating does not follow the commands.

What usually causes it?

Receiver relay has failed — thermostat sends signal but relay does not switch
Wireless signal not reaching the receiver from the thermostat
Incorrect wiring during installation
Motorised valve stuck or failed (not opening when called)
Boiler not responding to the call for heat
Thermostat paired to the wrong receiver or zone

Is it dangerous?

Not dangerous, but leaving heating uncontrolled can waste significant energy if the heating runs constantly, or cause comfort issues if it does not come on when needed. In very cold weather, lack of heating could lead to frozen pipes.

Can I fix it myself?

Check the receiver unit near the boiler — when the thermostat calls for heat, the receiver should click and a light should come on. If the receiver does not respond, try re-pairing the thermostat and receiver following the manufacturer's instructions. Check that the boiler is set to accept thermostat control rather than continuous operation.

When should you call an electrician?

Call an electrician if re-pairing does not fix the issue, if the receiver clicks but the heating does not start, if the system worked before and has stopped without any changes, or if the thermostat was recently installed.